While Hound and Sideswipe do have similar looking faces, I'm betting it's Sideswipe. The Perceptor head has a flat, oval shaped face just like Sideswipe, vs. Hound which is angled outwards along a line right below the nose. Also, the chin shape is slightly different (taking into account that the Perceptor head has it filed down). Sideswipe's nose starts angling out below his eyes, and while the Perceptor nose is filed down as well, there's a slight mark indicating where Sideswipe's nose would've started. Hound's nose, in contrast, is one piece and starts between his eyes. Hound also has deeper eyehole cuts; at the angle in the picture, you could confuse the two, but viewing Sideswipe's eyes at an angle produces the same effect (where there are shadows below the slit eyes angled outward, from the face sculpt).